Episodic post-rift subsidence of the United States Atlantic continental margin
PAIII I HFI I FR* ) r, " > U.S. Geological Survey, 345 Middlefield Road. Menlo Park, California 94025 CAKL M. WcN I WOK I H ) C. WYLIE POAG U.S. Geological Survey, Woods Hole, Massachusetts 02543
ABSTRACT study of the Continental Offshore Stratigraphie Test (COST) wells drilled on the United States Atlantic outer continental shelf and Sediment thickness, paleobathymetry, and chronostratigraphy slope off New Jersey (COST B-2 and B-3 wells) and Georgia (COST from COST wells offshore from Georgia and New Jersey indicate GE-1 well) has produced lithologie, chronostratigraphic, and periods of rapid subsidence superimposed on the slower thermal paleobathymetric information in sufficient detail for analysis of subsidence of the continental margin. Rapid subsidence occurred subsidence history (Poag and Hall, 1979; Poag, 1980). Inouranaly- during the Coniacian-Santonian, the Eocene and, in the COST wells off New Jersey, since the end of early Miocene. Once the maximum effects of water and sediment loading, compaction, and thermal cooling are removed, the residual vertical movements caused by tectonic and sea-level fluctuations can be analyzed. Because no global sea-level change can account for all residual movements, we propose that tectonism, variously amplified by loading, is responsible for the observed episodes of rapid subsi- dence. Synchroneity of subsidence with sea-floor-spreading changes in the North Atlantic suggests a unified cause for these events. Recognition of episodic subsidence may have implications for fault timing, petroleum potential, and global sea-level effects on passive margins.
INTRODUCTION
The Atlantic margin of North America is generally thought to have subsided regularly as a result of cooling and sediment loading ever since continental rifting occurred in the early Mesozoic (Sleep, 1971; Walcott, 1972; Watts and Ryan, 1976; Steckler and Watts, 1978; Keen, 1979; Royden and Keen, 1980). However, studies of sedimentation rates from wells on the Atlantic Coastal Plain in North Carolina and on the offshore margin of Georgia and New Jersey (Rona, 1973; Whitten, 1976a, 1976b, 1977; Poag and Hall, 1979; Poag, 1980) suggest that the subsidence history was irregular in the Cretaceous and Tertiary. The geologic record from which subsidence history can be reconstructed is the sedimentary section that accumulates as subsidence proceeds. Not only age and thick- ness of sedimentary.units, but also detailed paleobathymetry must be known, because ancient water depths can be great and quite variable. The combination and graphical presentation of such data, as described by van Hinte (1978), produce an especially useful por- trayal of subsidence history. This study demonstrates that at least two widely separated sites along the continental margin of the eastern United States have had 72" W nearly synchronous episodes of rapid subsidence superimposed on Figure 1. Eastern United States continental margin, showing the slow thermal subsidence presumed for the rifted margin. Recent coast-line configuration, COST well locations, major structural elements, coastal-plain boundary, depth to Paleozoic basement *Present address: Laboratory of Geotectonics, Department of Geosci- (after King, 1969), and continental, transitional(?), and oceanic- ences, University of Arizona, Tucson, Arizona 85721. crust boundaries (after Klitgord and Behrendt, 1979).
Geological Society of America Bulletin, v. 93. p. 379-390, 6 figs., 1 table. May 1982.
379
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sis, we examine the total subsidence history from this well informa- We treat the eastern United States as a simple, subsiding, pas- tion, adjust for sediment compaction, then consider the contribu- sive margin, because the regional tectonic regime has been consist- tions of sediment and water loading, lithospheric cooling, other ent since rifting. Basins of late Mesozoic and Cenozoic age along tectonic events, and global sea-level fluctuations. The resultant sub- the continental margin (Southeast Georgia Embayment, Salisbury sidence and residual curves, supported by structural observations, Embayment) overlie inferred Triassic grabens and lie landward of from onshore Georgia, document the timing, duration, and magni- the larger marginal troughs (Blake Plateau Basin, Baltimore tude of episodic subsidence. Canyon trough) that formed on transitional crust during and soon after rifting (Klitgord and Behrendt, 1979). The area between the SETTING embayments has persisted as a positive feature (Carolina Platform, Cape Fear Arch). In addition, deformation of sea-level indicators The United States Atlantic margin is a passive margin of con - across the Southeast Georgia Embayment (Winker and Howard, nected continental, transitional, or rift-stage type, and oceanic crusl: 1977; Cronin, 1980) and the embayment of the present shoreline at (Fig. 1; Klitgord and Behrendt, 1.979), that extends more than 2,000 the sites of the late Mesozoic and Cenozoic basins suggest that these km from Florida to coastal Canada. The margin is mantled by a areas have continued to be tectonically active over the past 3 m.y. wedge of mostly shallow-water marine and nonmarine sedimentary Because of their thick sedimentary sections and petroleum deposits, accumulated during and after rifting, that extend 200 to potential, these basins have been more thoroughly studied than the 500 km across the Atlantic Coastal Plain and continental shelf and intervening platform (Schlee and others, 1976; Poag, 1978; Dillon slope. From their landward limit, these deposits thicken eastward to and Paull, 1979). Since the two B-wells are only 52 km apart, the as much as 18 km in the deep continental-margin basins (Folgerancl three COST wells used in this study establish essentially two data others, 1979; Grow, 1980). points, about 1,200 km apart, on the continental margin.
TABLE 1. COST WELL DATA
Top of Age SS *
COST GE-1 1 108 1093 1093 .11 0 0 0 0 2 103 1369 1380 .13 0 20 83 86 3 100.5 1480 1493 .13 20 100 128 183 4 99.5 1579 1599 .13 0 20 147 161 5 91 1622 1643 .14 0 20 159 173 6 90 1694 1721 .14 20 100 198 253 7 85.5 1694 1721 .14 0 20 184. 198 8 84 1839 1870 .15 0 20 228 241 9 82 1974 2013 .16 20 100 286 342 10 79 2079 2115 .17 100 200 368 436 11 65 2244 2277 .18 200 500 481 688 12 64 2277 2311 .19 100 200 422 491 13 59 2277 2311 19 20 100 367 422 14 58 2294 2328 .19 20 100 372 428 15 55 2346 2378 .20 100 200 441 509 16 52 2346 2378 .20 20 100' 386 441 17 45 2599 2621 .22 20 100 453 508 18 40.5 2854 2867 .24 100 200 579 648 19 39.5 2947 2957 .25 20 100 551 606 20 39 2977 2986 .26 100 200 615 684 21 34 2977' 2986 .26 200 500 684 890 22' 28 3134 3138 .28 200 500 731 938 23 12 3184 3187 .29 200 500 747 953 24 0 3279 3279 .30 20 100 653 708 COST B-2 1 135 9312 9312 .08 0 0 0 0 2 107 10205 10344 .10 0 0 359 359 3 105 10388 10523 .20 0 0 409 409 4 95 10499 10624 .20 50 150 466 535 5 92 10571 10690 .20 100 200 515 584 6 91 10571 10690 .20 0 20 446 460 7 89 10636 10760 .22. 0 20 472 486 8 88 10784 10938 .25 20 100 563 618 9 85 10784 10938 .25 0 0 549 549 10 82.5 11084 11228 .30 0 0 642 642 11 81 11104 11246 .32 20 100 661 716 12 72 11225 11378 .32 100 200 770 839 13 70 11286 11439 .32 50 150 757 826 14 68 11359 11503 .32 0 20' 739 753 15 55 11359 11503 .32 150 250 842 911 16 51 11406 11543 .35 200 500 888 1094
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ANALYSIS OF SUBSIDENCE interpretation of microfossils. Subsidence is presented in positive values and uplift in negative values. The total subsidence relative to sea level of a stratigraphic Use of present sea level as a datum creates complications, horizon during an interval of time amounts to the sum of the net because past variations in sea level cannot be directly distinguished change of water depth and the thickness of sediment accumulated from subsidence. Either a rise in sea level or an increase in subsi- over the time interval, corrected for any postdepositional compac- dence would appear in the stratigraphic record as an increase in tion (van Hinte, 197.8). Repeating the calculation over selected water depth and/or in sediment thickness. Incorporation of sea- increments of time since the horizon was formed allows one to level history might resolve these complications, but proposed sea- construct a curve that portrays the total subsidence history. The level curves differ significantly, particularly in absolute magnitudes subsidence curve for any horizon, therefore, begins at the age and of eustatic changes. The Late Cretaceous high stand, for example, is depth of deposition of that horizon, passes through points that variously reported to have reached about 150 m (Watts and represent cumulative subsidence of that horizon during intervening Steckler, 1979), about 350 m (Vail and others, 1977; Pitman, 1978), intervals of time, and ends at the depth at which the horizon is and more than 600 m above present sea level (Hancock and Kauff- found today. The smaller the time intervals used, the more detailed man, 1979). Rather than incorporate a proposed absolute global the resultant subsidence curve. Data needed for calculations of sub- sea-level curve in the calculation of the subsidence curves, we first sidence (namely, sediment thickness over a time interval, age, and calculate total subsidence curves, and then we discuss the effects of paleobathymetry) are determined fr.om microfossils. The datum for possible sea-level variations separately. the calculated subsidence curves is sea level, which has varied inde- The occurrence of unconformities in the stratigraphic section pendently over time. We consider sea-level changes as changes in does not preclude the reconstruction of the subsidence curve. Sub- datum only and treat paleobathymetry separately, as based on sidence during every time interval is calculated independent of adja-
TABLE I. (Continued)
Top of Age SS *
17 49.5 11482 11610 .35 150 350 872 1009 18 46 1 1558 11676 .38 200 500 926 1132 19 41 11638 11747 .38 500 1000 1153 1497 20 38 11696 11806 .40 350 750 1074 1349 21 32 11696 11806 .40 150 350 936 1074 22 26 11830 11934 .40 150 350 983 1121 23 21 11848 11950 .40 10 50 892 919 24 14 11848 11950 .40 0 100 885 953 25 12 12461 12499 .45 0 100 1077 1146 26 0 12800 12800 .50 100 200 1258 1327 COST B-3 1 142 9763 9763 .05 0 0 0 0 2 131 10404 10469 .05 20 100 221 276 3 126 10684 10785 .10 0 20 312 326 4 124 10830 10922 .10 20 100 353 408 5 120 10830 10922 .10 0 0 339 339 6 119 10869 10959 .12 0 0 346 346 7 107.5 11120 11219 .16 0 20 425 438 8 102 11196 11298 .16 20 100 463 518 9 97 11440 11564 .17 0 20 539 553 10 94 11501 11619 .20 20 100 565 620 11 103.5 11529 11644 .20 60 150 598 660 12 91 11529 11644 .20 20 100 570 626 13 89 11634 11763 .27 20 100 619 674 14 85.5 1 1634 11768 .27 0 10 606 612 15 82.5 1 1885 12029 .30 10 60 .713 748 16 81 11989 12128 .30 100 200 806 875 17 72 12172 12316 .32 200 500 944 1151 18 68 12192 12334 .32 100 200 881 950 19 52 12233 12369 .33 100 200 889 958 20 48 12376 12503 .35 200 500 1002 1209 21 39 12492 12603 .37 800 1200 1443 1718 22 38 1.2516 12626 .38 500 1000 1245 1588 23 20 12660 12757 .40 500 1000 1289 1633 24 18 12672 12768 .40 200 500 1086 1292 25 12 L2858 12929 .44 200 500 1138 1344 26 0 13200 13200 .51 800 ,••800 1632 1632
S = cumulative sediment thickness in metres; S* = uncompacted cumulative sediment thickness in metres;
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cent time intervals. Details of subsidence history during a hiatus are (Berggren and Van Couvering, 1974; van Hinte, 1976b), uncertainty lost, but net subsidence is known, because the water depth at the in absolute age arises in correlation to different absolute time scales. end of the interval previous to the unconformity and the water Whitten (1977) has demonstrated that relative changes in calculated depth at the beginning of the subsequent interval are independently Cretaceous sedimentation rates differ only slightly when the time established. scale of Obradovich and Cobban (1975) is substituted for that of Errors generated by wide sample spacing, by uncertainties in van Hinte (1976b). paleobathymetry and chronostratigraphy, and to some extent by Paleobathymetric estimates are based chiefly on comparing the the omission of sea-level changes in the curve construction can be fossil foraminiferal assemblages with aspects of the community minimized by using only wells that penetrate thick, rapidly depos- structure of modern benthic foraminiferal assemblages, specifically, ited sedimentary materials. In this study, therefore, only the COST generic predominance among benthic forms, species diversity, and wells are used. Onshore wells are not used because sedimentary absolute abundance of individuals. This procedure leads to esti- units are thinner and sections are less complete, owing to the pres- mates of the approximate depths at which the fossil assemblages ence of unconformities. They also penetrate more nonmarine de- would have lived under the environmental constraints of today's posits, which offer poorer biostratigraphic and paleobathymetric western North Atlantic Ocean. Lithologic aspects of the rocks, such control than the offshore wells. as the presence: of oolites, glauconite, or coal, were also used, espe- cially where microfossils were not recovered. Data Sources Although estimation of paleodepths from foraminiferal assem- blages is a well-established technique, uncertainties in the relations Data for our subsidence analysis come from three COST wells between the organisms and ancient oceanic conditions preclude (Fig. 1; Table I) that penetrated a thick section of postrift sediments anything better than approximations of paleodepth. The bathymet- accumulated in the continental-margin basins. The oldest sediments ric distribution of modern benthic foraminifera can be correlated penetrated were of Early Cretaceous or Late Jurassic age (Poag, with a host of abiotic environmental factors, including salinity, 1980; Poag and Hall, 1979). The COST B-2 well was drilled in the temperature, oxygen content, substrate composition or grain size, Baltimore Canyon trough to a total depth of 4,864 m below sea sunlight penetration, availability of calcium carbonate, and envi- level (Poag, 1980). Total sedimentary-section thickness here is 12.8 ronmental stability (Douglas, 1979; Poag, 1981). None of these km or more, below which lies seismic basement interpreted as tran- properties need maintain a constant relation to depth through geo- sitional crust (Mattick, 1977; Grow and others, 1979). The COST logic time. There is evidence that hydrostatic pressure does control B-3 well reached a total depth of 4,810 m below sea level (Poag, certain enzymes and affects cell walls and nerve-cell functions in 1980). Seismic basement here is considered to be oceanic crust and marine organisms (J. W. Valentine, 1979, personal commun.), but it lies at a depth of about 13.2 km (Grow, 1980). The COST GE-1 well has not been conclusively demonstrated that benthic foraminiferal in the Southeast Georgia Embayment was drilled over a basement distribution is directly controlled by water depth. ridge to a depth of 4,010 m below sea level, penetrating continental Our paleobathymetric estimates contain the internal consis- basement composed of Paleozoic rocks at about 3,280 m (Simonis, tency provided by the species identifications and interpretations of 1979). a single paleontologist. Other paleobathymetric estimates on the Poag and Hall (1979) and Poag (1980) have interpreted the same wells, using different criteria by other workers, vary from ours chronostratigraphic record and paleobathymetry of these wells in detail, but generally show the same major fluctuations, with chiefly from planktonic and benthic foraminiferal assemblages water depths between our minimum and maximum values (see observed in rotary ditch cuttings collected at 3- to 27-m intervals, Lachance and Steinkraus, 1978; Bebout and Lachance, 1979; Val- and from sidewall core samples taken at irregular intervals. As entine, 1979, 1980). such, this data base is more refined than those used in previous Possible errors generated during the assignment of numerical paleobathymetric studies in these wells, including that used in water depths to paleobathymetric interpretations should have only Steckler and Watts' (1978) subsidence analysis of the COST B-2 a slight effect on the constructed subsidence curves, because (1) our well. Since the uppermost 300± m of each well was not sampled, the depth assignments tend to be somewhat shallower than some pro- youngest dated samples are of middle Miocene age, and the post- posed (see van Hinte, 1978; Ingle, 1980), minimizing the effect of early Miocene section is therefore treated as a single interval. bathymetry; (2) in these wells, the change in water depth over time The biostratigraphic zonations are based primarily on plank- is minor when compared to sedimentation rates; (3) the paleo-water tonic foraminifers recovered in the Upper Cretaceous and Cenozoic depths in these wells are generally quite shallow, rarely deeper than rocks, but radiolarians provide secondary control in the silica-rich upper slope; and (4) although in some wells the subsidence history is Miocene rocks. Benthic foraminifers supplemented by ostracodes recorded largely by paleobathymetric changes, and in others it is and palynomorphs provide the basis for age estimates for the Lower recorded largely by sediment accumulation, the general history of Cretaceous and Jurassic rocks. For the Cenozoic, foraminiferal total subsidence is similar in timing and amount for all three wells. zonations and time scales used are those of Blow (1969), Berggren The abrupt steps in the subsidence curves are somewhat artificial and Van Couvering (1974), Stainforth and others (1975), and Har- and result from the use of discrete water-depth categories. denbol and Berggren (1978); for the Cretaceous and Jurassic, zones Throughout the analysis, we have preferentially chosen conserva- and time scales of Pessagno (1967), Ascoli (1976), and van Hinte tive values, to minimize the residual component of tectonic/sea- (1976a, 1976b) are used. The radiolarian interpretations are based level subsidence. on the biostratigraphy of Kling (1978) and Riedel and Sanfilippo (1978). Palynomorph interpretations are those of Williams (1975), TOTAL SUBSIDENCE CURVES Steinkraus (1979), and Bebout (J. W. Bebout, 1979, personal com- mun.). Although planktonic foraminiferal zonations for the Late We have reconstructed subsidence histories in the three COST Cretaceous and Cenozoic have a resolution of roughly 1 m.y. wells by tracking the deepest horizons for which detailed paleonto-
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AGE (m.y. B P.)
COST < 2 — r B-2 < 2- z£ ui < 2 o o = 2? a z z 2 < m > i m 3 o < < < i i i i i< l,-l° if I ° Is tfl
150
~r 100 AGE (m.y. B.P.) AGE (m.y. B.P.)
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logic control exists (Fig. 2): the bases of the Albian, Berriasian, and of the steps in the curves results, in part, from changes in our late Kimmeridgian stages, respectively, for the COST GE-1, COST paleobathymetric estimates, the steplike character of the bathymet- B-2, and COST B-3. ric scale, and the spacing of our sampling interval; in reality, the Correction for compaction was tested for each well and found transitions are probably smoother. to have very minor effect on the shape of the total subsidence curves The total-subsidence curves plotted in Figure 2 show very sim- (Fig. 2). Even for the shale-rich COST B-3 section, compaction ilar histories, two of the three major subsidence events being similar accounted for less than 25% of the total subsidence curve at any in timing, duration, and magnitude in all three COST wells. point in time and has little effect on the shape of the curve. Marked periods of rapid subsidence occurred during the Coniacian- Compaction corrections were made, following the method of Santonian and in the Eocene. A post-early Miocene event is also van Hinte (1978), on all clayey intervals above the tracked horizon . evident in both B-wells, but it is not found in the GE-1; a less- Any compaction beneath that horizon would affect only the earliest prominent Aptian-Albian event may also exist. part of each subsidence curve, because most compaction occurs Of the three major subsidence events, the Eocene event seems within the first few hundred metres of burial. The corrections were to be the most profound and can be documented onshore by con- limited to clay and shale intervals, because carbonate sediments temporaneous events. Plots of generalized dip angle and strike mostly compact soon after deposition (less than 106 yr; Weller, direction relative to time for the Coastal Plain formations of west- 1959; Schinn, 1969; Schinn and others, 1977; Schinn and Robbiri, central Georgia (Vorhis, 1974, Figs. 2, 3) show a definite increase in 1980) and sandstones do not compact very much (commonly less the rate of change of strike rotation during the Eocene ascribed to than 20%; Welier, 1959; Perrier and Quiblier, 1974). Any interval an increase in rate of depocenter migration during this time. In reported to be dominantly clay or shale (Rhodehamel, 1977a, 1979; constructing similar plots to examine the rate of change of gener- Pollack, 1980) was considered to be entirely of that composition, alized dip of formations in eastern Georgia from published well thus maximizing that correction. Porosity change, or compaction, data, we find a distinct increase in rate of dip change during the with increasing depth of burial is modeled using: Eocene (Fig. 3). This change may result from an increased rate of eastward tilting during this epoch caused by an increase in rate of = «¿max e-°'45 D (!) subsidence eastward toward the continental margin.
(Ozerskaya, 1965; Rieke and Chilingarian, 1974), where 4> is poros- SUBSIDENCE MECHANISMS
ity at depth D (in km) and max is maximum initial porosity of the clayey sediment. This curve is constrained by a single downhole The total-subsidence curves shown in Figure 2 may be the porosity measurement for each of the COST wells (Rhodehamel, result of several distinct processes: (1) progressive sediment and 1977b; Amato and Bebout, 1978; Amato and Simonis, 1979). water loading of the crust with attendant isostatic response, (2) The total-subsidence curves show generally decreasing rates of postrift cooling and densification of the lithosphere, and (3) other subsidence with time, as would be expected at a passive margin for tectonic changes. We have already corrected for compaction, find- which cooling and thickening of the lithosphere is the primary sub- ing that it had little effect on the shape of the subsidence curves. sidence mechanism (Sleep, 1971; Stecklerand Watts, 1978; Royden Possible fluctuations in global sea level, however, have not yet been and others, 1980). They also show a very prominent steplike pat- considered. To analyze the effects of each of these components on tern, however, which indicates that subsidence relative to present the subsidence history, we will first calculate and remove the maxi- sea level has been episodic, a behavior not accounted for by simple mum possible effects of loading, yielding a tectonic/ sea-level curve. cooling. The steps involve episodes of rapid subsidence which Next we will remove the effect of a simple cooling history from this endured less than 10 to 15 m.y,, during which rates exceeded 50 :o curve; then, we will examine the residuals in the light of possible 100 m/m.y. These were separated by periods of much slower subsi- sea-level fluctuations and any remaining tectonic components. dence, or even uplift, lasting 20 to 30 m.y. The rapid-subsidence events account for 50 to 95% of the total observed subsidence in Loading only one-third or less of the total time. The smaller variations along the curves may have some significance, but the errors inherent in Isostatic considerations require that, given an initial water the method make them more difficult to evaluate. The abruptness depth, subsidence due to sediment infilling and loading cannot
Figure 3. Change of dip of formation tops relative to age for the Atlantic Coastal Plain of Georgia. Dip values represent the slope of formation tops between published wells (Herrick, 1961; Maher, 1971) located down regional structure from each other, as well as generalized dips given by Herrick and Vorhis (1963). Line connects mean dip value; one standard deviation (vertical bar) and number of data points for each horizon are also shown. AGE (mybp)
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1/2 1/2 1 2 TIME TIME TIME '
8 10 12
COST B-3 Figure 4. Tectonic/sea-level subsidence relative to the square root of time since rift- ing (m.y. B.P.). Rifting taken as 175 m.y. I t- 500 500 B.P. (Smith and Noltimier, 1979). CL HI O
1000
1500 1500
exceed about 2.5 times the initial depth. Any further subsidence must be produced by other mechanisms. A maximum estimate of loading effects can be made by balancing a lithospheric column, Since much of the total subsidence is driven by sediment load- with and without the load, in a simple local-loading (Airy type) ing, varying the rate of sedimentation will necessarily affect the model (Watts and Ryan, 1976; Steckler and Watts, 1978). Remov- subsidence rate. This explains why the total-subsidence curves are ing the effects of loading or backstripping (Watts and Ryan, 1976) greatly flattened during unconformities. Backstripping removes all by this method yields the remaining minimum amount of subsi- sediment-loading effects, however, leaving the residual tectonic/ dence due to tectonic mechanisms and sea-level fluctuations. sea-level curve unaffected by unconformities. In backstripping, as done by Steckler and Watts (1978), water- loading was assumed constant, because they were comparing the Lithospheric Cooling subsidence of the submerged continental margin with that of the oceanic crust. Here, however, to produce a conservative estimate of Recent studies of subsidence of Atlantic-type margins suggest episodic subsidence, we have considered the loading effects of both that, during postrift cooling and densification, the crust subsides sediment and water in backstripping the total-subsidence curves, first as the square root of time and then as an inverse exponential using the method described in the Appendix. This procedure results with time (Sleep, 1971; Steckler and Watts, 1978; Keen, 1979; in a minimum tectonic/sea-level component (Y'), shown in Figure Royden and others, 1980). In the case of the post-Jurassic history of 2. Backstripping using an Airy model maximizes the loading effect, the three COST wells, a simple curve that best fits the tectonic/sea- because, in reality, the lithosphere has some flexural rigidity that level component of subsidence in each well follows the form of the provides lateral strength to support some of the load (Walcott, square root of time (Fig. 4). These best-fit curves are derived using 1972; Watts and Ryan, 1976; Steckler and Watts, 1978). 175 m.y. B.P. as the initiation of subsidence, when spreading began Any one of an assortment of errors intrinsic to an analysis such in the central Atlantic Ocean (Smith and Noltimier, 1979). We as this may alter the absolute values of subsidence through time. assume, therefore, that a curve of this form represents cooling sub- These errors include the choice of constants, equations for back- sidence of the continental margin. stripping and compaction corrections, and scales for time and paleobathymetry. We have run our data several ways with different Sea-Level Fluctuations constants, equations, and computational schemes; resultant curves, although slightly different in absolute magnitudes, are very The effect of eustatic fluctuations of sea-level must be consid- similar in form, and all contain the same relative changes of epi- ered: changes in sea-level datum of the order of hundreds of metres, sodic subsidence. such as is thought to have occurred during the Tertiary (Vail and The backstripped curves (Y', Fig. 2), representing the tectonic/ Hardenbol, 1979), would have a major effect on the subsidence sea-level component of subsidence, retain much of the shape of the curves. To test the role of eustatic fluctuations, we must examine total-subsidence curves, but they are reduced by a factor of about the possibility that the deviations of the tectonic/sea-level curve two. The major rapid-subsidence events are still clearly evident, about the expected simple cooling curve (Fig. 5b) represent global indicating that loading has amplified, but is not solely responsible sea-level changes (Watts and Steckler, 1979). for, these events. The Aptian-Albian event, relatively small to begin Unfortunately, present knowledge of global sea-level changes with, is nearly obliterated by backstripping and cannot be distin- is not sufficient to preclude eustasy as the sole cause of apparent guished from other subordinate features on the curves. This event is episodic subsidence. There are, however, lines of evidence that sug- either driven entirely by sediment loading, or its tectonic/sea-level gest that sea-level changes cannot account for most of our obser- component is overwhelmed as the effects of loading are maximized. vations and are, therefore, of secondary importance: (1) The
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magnitude and timing of the residual curves in Figure 5b bear little AGE (mybp)
resemblance to published global sea-level curves or to other eustatic 140 120 100 80 60 40 20 sea-level indicators (Fig. 5a). Either these published curves, all of (a) which agree in shape though not in magnitude, are grossly in error, or the residual curves reflect major tectonic influence as well as sea-level changes. (2) The magnitude of the middle Eocene sea-level rise required by the curves in Figure 5b, in particular, is too large. A >11 1 sea-level rise of between 350 and 700 m, at a rate between 3 and 6 UJ cm/ka during this period, seems unreasonable when conventional y r L -200 mechanisms of eustatic sea-level change—glaciation, collision orog- 1 eny, sediment filling of ocean basins, Mediterranean-type basin flooding, hot-spot formation, and changes in ridge spreading rate—are considered (W. C. Pitman, III, 1980, oral commun.). Further, (3) the timing and magnitude of change for each well differ 1 sufficiently that no single sea-level curve can satisfy them all. In the most extreme example, the COST B wells suggest a sea-level rise 1 during Neogene time, whereas the GE-1 well requires a sea-level COSI GE-1 fall. And, the Eocene event in the B wells would require a sea-level 1 rise of at least 350 m and initiated as much as 10 m.y. earlier than h. , / \ the GE-1 well, which needs less than a 350-m sea-level change. J !/ Global sea-level changes should affect every margin a nearly equal amount, simultaneously, whereas tectonic events can vary in timing 1 \ +600 and magnitude across large distances. Errors should not represent internal inconsistency in relative timing and paleobathymetry between wells, because all microfossils were analyzed by a single paleontologist.
Tectonism
Our inability to attribute the tectonic/sea-level residuals of Figure 5b mainly to sea-level fluctuations requires that in signifi- cant part they be tectonic. This means that penecontemporaneous tectonic subsidence events have occurred along the Atlantic margin at the latitudes of Georgia and New Jersey during both the Coniacian-Santonian and the Eocene. The Eocene event is larger and is evident onshore in Georgia and probably as far north as Canada. Large positive deviations (that is, subsidence) from the expected thermal-subsidence curve on the Labrador and Nova Sco- tian shelves of Canada (Keen, 1979; Royden and Keen, 1980) may represent this same event. The late Cenozoic event off New Jersey is not evident in the GE-1 well, where modest uplift occurs during the saitìjH i f ^ 'ILES £ luy same period. ri't-Sr'^B tela. li.* pu><^ m X< ffl< Ollj =3 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/5/379/3444567/i0016-7606-93-5-379.pdf by guest on 30 September 2021 SUBSIDENCE OF UNITED STATES ATLANTIC CONTINENTAL MARGIN 387 A relationship between subsidence and relative motion of the NORTH ATLANTIC SUBSIDENCE (cm/ka) North American plate appears to exist, on the basis of observation HALF-SPREADING DIRECTION RATE of synchroneity between the episodic subsidence and major changes COST GE-l COST B-2 COST B-3 in North Atlantic spreading rates and direction as presently under- > g (cm/yr) S5 ¡S 0 12 10 0 5 10 10 stood (Fig. 6). Changes in trend of the Kane Fracture Zone (about „ , | i I j. I I J_ 24° N latitude) may indicate a change in the direction of North Atlantic spreading (Rabinowitz and Purdy, 1976; Rona and Richardson, 1978; Purdy and' others, 1979). Spreading prior to V-i.- anomaly 34 (about 81 m.y. B.P.) was-essentially parallel to today's direction. Between anomalies 34 and 31 (about 73 m.y.), spreading began to change to a more southwesterly direction and remained in that orientation until, between anomalies 25 (about 63 m.y. B.P.) and 21 (about 53 m.y. B.P.), it changed back to the present spread- ing direction. The period between 81 and 53 m.y. B.P. may also ] have been a time of increased half-spreading rate, as also the past 9 m.y. may. have been. Changes in spreading rate and direction appear to be synchro- PQ nous with episodic subsidence (Fig. 6), although a positive change >> 50 in spreading rate apparently may be related to either an increase or decrease in subsidence rate. Some spreading changes occur at the J. start of episodes of rapid subsidence; others mark the end of an W episode. Therefore, although there is a temporal relationship < between North Atlantic spreading and subsidence of the continental margin, this relationship is not consistent with regard to rate or direction. This observation contrasts with the consistent, direct correlation between sea-floor spreading and global sea-level fluc- tuations (Pitman, 1978). Uncertainties that exist in the spreading rates as a result of poorly constrained anomaly ages (LeBrecque and others, 1977; Ness and others, 1980) and time scales (Baldwin and others, 1974) may affect our correlations. The causal mechanism for these episodes is not known. The evidence for shallow igneous intrusions in Virginia during the 100 - Eocene (Fullagar and Bottino, 1969) and the abundance of chert and altered volcanic glass in Eocene sedimentary rocks along the Atlantic margin (Gibson and Towe, 1971) suggest that subsidence during this time might be related to a widespread pulse of thermal activity. The large distance separating the COST wells (1,200 km), however, is at the extreme limit of size expected to be affected by a Figure 6. Timing of subsidence events on the U. S. continental single hot-spot event (Crough, 1978). Moreover, any lithospheric shelf relative to changes in North Atlantic spreading. COST wells heating would require uplift nearly equal in amount to, and imme- total-subsidence rates shown, in cm/ka, calculated using median diately preceding, the rapid-subsidence events. There is no record of paleo-water depths. Negative values, denoting uplift, shown to the such major uplift in the COST wells or in the many seismic profiles left of the zero line. North Atlantic half-spreading rates from Pit- along the United States Atlantic continental shelf (W. P. Dillon, man and Talwani (1972), and compilation by Coney (1973); 1980, written commun.). The Neogene event, seen only in the spreading directions are determined from the trend of the Kane northern wells, may have resulted from the tectonic effects of load- fracture zone (Purdy and others, 1979). ing by ice sheets during the Pleistocene (Walcott, 1970), as has been suggested for similar events on the Canadian shelf (Royden and Keen, 1980). Other mechanisms that might account for episodic expect that episodic subsidence occurs on other passive margins, subsidence include crustal or lithosphere-asthenosphere phase although perhaps with different timings. Certainly, the possible changes (Falvey, 1974; Sloss and Speed, 1974; Neugebauer and existence of such events must be considered in any passive-margin Spohn, 1978) and changes in stress regime along the continental analysis, including global sea-level reconstruction (Vail and others, margin (Bott, 1971, 1979; Artyushkov, 1973) due to changes in 1977; Vail and Hardenbol, 1978; Watts and Steckler, 1979). ridge-push forces (Neugebauer and Spohn, 1978; Richardson and others, 1979). Although our observations have shown a temporal ACKNOWLEDGMENTS relationship with spreading changes in the North Atlantic, we can- not evaluate the ultimate driving force of subsidence. We have benefitted from discussion with many colleagues at Episodic subsidence may tell us something about periodicity of the U.S. Geological Survey, the American Association of Petrol- other events along the United States Atlantic continental margin, eum Geologists research meeting on Interregional Unconformities such as the timing of movements on faults (York and Oliver, 1976) in 1980, the Laboratory of Geotectonics at the University of Ari- and increased oil maturation in young sediments during periods of zona, and elsewhere. In particular, we would like to thank G. C. rapid subsidence (Royden and others, 1980). On a larger scale, we Bond, W. R. Dickinson, W. P. Dillonj Larry Mayer, M. Mergner- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/5/379/3444567/i0016-7606-93-5-379.pdf by guest on 30 September 2021 388 HELLER AND OTHERS' Keefer, R. M. Richardson, M. L. Sbar, M. S. Steckler, A. Swift. ments Special Publication, v. I, pt. B., p. 653-677. A. B. Watts, J. C. Yount, P. R. Vail, and M. L. Zoback for their Baldwin, B„ Coriey, P. J., and Dickinson, W. R., 1974, Dilemma of Cre- generous criticisms and comments. Computer assistance was gra- taceous time scales and rates of sea-floor spreading: Geology, v. 2, p. 267-270, ciously provided by Larry Mayer and M. Mergner-Keefer. Bebout, J. W., and Lachance, D. J., 1979, Depositional environments, in Amato, R. V., and Simonis, E. K., eds., Geological and operational APPENDIX 1. CALCULATION OF SUBSIDENCE summary, COST No. B-3 well, Baltimore Canyon Trough area, mid- Atlantic OCS: U.S. Geological Survey Open-File Report 79-1159, p. 40-48. Assuming a simple local-loading (Airy type) model, the amount of Berggren, W. A., and Van Couvering, J. A., 1974, The Late Neogene — subsidence over an interval of time due to loading by sediment and water Biostratigraphy, geochronology, and paleoclimatology of the last 15 can be determined. This leaves a residual amount attributable to tectonic: million years in marine and continental sequences: Palaeogeography, subsidence and sea-level changes. Subsidence is derived, after van Hinte Palaeoclimatology, Palaeoecology, v. 16, 216 p. (1978) and Steckler and Watts (1978), as follows: Blow, W. H., 1969, Late middle Eocene to Recent planktonic foraminiferal biostratigraphy, in Proceedings of the First International Conference T = S*+W , (2) s d on Planktonic Microfossils, Geneva, 1967: Leiden, Brill, v. 1, p. 199-421. where T is total subsidence, S* is total sediment thickness corrected for s Bond, G., 1978, Speculations on real sea-level changes and vertical motions compaction, and W is water depth. Total subsidence is also equal to the d of continent s at selected times in the Cretaceous and Tertiary periods: sum of the sediment load, water load, and tectonic subsidence, minus the Geology, v. 6, p. 247-250. change of sea level, expressed as: Bott, M.H.P., 1971, Evolution of young continental margins and formation of shelf basins: Tectoriophysics, v. 11, p. 319-327. Ts = S*(ps/p,„) + Wd(pw/pm) + Y', (3) 1979, Subsidence mechanisms at passive continental margins, in Wat- kins, J. S., Montodert, L., and Dickerson, P. W., eds., Geological and where Y' = Y - ASL, Y is tectonic subsidence, ASL is change in sea level, Y' geophysical investigations of continental margins: American Associa- is tectonic-sea-level subsidence, p is average sea-water density (1.03), and w tion of Petroleum Geologists Memoir 29, p. 3-9. p is average mantle density (3.3). The value of p , density of sedimen: m s Coney, P. J., 1973, Noncollision tectogenesis in western North America, in column, is represented by Tarling, D. H., and Runcorn, S. K.., eds., Implications of continental drift to the earth sciences: London, Academic Press, p. 713-727. . K [1»i(Pw) + (l-t»i)(Pg)] Tj Cronin, T. C., 1980, Biostratigraphic correlation of Pleistocene marine de- posits and sea levels, Atlantic Coastal Plain of the southeastern United States: Quaternary Research, v. 13, p. 213-229. (after Steckler and Watts, 1978), where subsidence curves (Ts) used for the calculation of tectonic/sea-level subsi- Short Course No. 6, Houston, Texas, 1979, p. 21-53. dence (Y-) were chosen such that they would produce the most conservative Falvey, D. 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